The present invention relates to endovascular cryocatheters, such as angioplasty balloons having a freezing function for treating tissue by extreme cooling contact. These catheters have an elongated body through which a cooling fluid circulates to a tip portion which is adapted to contact and cool tissue. Such a device may include a steering assembly such as an inextensible pull wire and a flexible tip to which the pull wire attaches which may be bent into a curved configuration to aid its navigation through blood vessels to a desired treatment site. When used for angioplasty or the destruction of tissue on the inner wall of a vessel, the catheter generally also has one or more inflatable balloon portions which may serve two functions of displacing blood from the treatment site to allow more effective cooling, and physically distending the affected vessel to break up accumulations of plaque.
Endovascular catheters must be of relatively small diameter, and configured for insertion along relatively confined pathways to reach an intended ablation site. As such, the cooling fluid must circulate through a relatively long and thin body yet apply significant cooling power in their distal tip. The requirement that coolant be localized in its activity poses constraints on a working device. For example, when the catheter must chill tissue to below freezing, the coolant itself must obtain a lower temperature to offset the conductive warming effects of adjacent regions of body tissue. Furthermore, the rate of cooling is limited by the ability to circulate a sufficient mass flow of coolant through the active contact region. Since it is a matter of some concern that proximal, adjacent or unintended tissue sites should not be exposed to harmful cryogenic conditions the flowing coolant must be exposed in a limited region. One approach to cooling uses a phase change refrigerant which is provided through the body of the catheter at relatively normal or ambient temperature and attains cooling only upon expansion within the tip region. One such device treats or achieves a relatively high rate of heat transfer by using a phase change coolant which is pumped as a high pressure liquid to the tip of the catheter and undergoes its phase change expanding to a gas in a small chamber located at the tip. The wall of the chamber contacts the adjacent tissue directly to effect conductive cooling or ablation treatment. Other cryocatheters may employ gas at high pressure, and achieve cooling via the Joule-Thomson effect at a spray nozzle in a cooling chamber at the distal end of the catheter.
In an endovascular catheter as described above, a relatively high cooling power may be obtained. However, the expansion of a phase change or high pressure coolant exiting from a nozzle within a small catheter tip creates highly turbulent flow conditions. The cooling region of the tip may be implemented as a fairly rigid chamber having highly thermally conductive wall or section of its wall formed for example by a metal shell. However, if one were to replace such a tip with an inflatable balloon as is commonly used for angioplasty, the size of the chamber would vary considerably as the balloon is inflated, causing substantial variations in flow conditions of the fluid entering the tip and substantial changes in heat transport across the expanding balloon wall. Both of these factors would result in variations of the cooling power over the tip. Furthermore, coolant materials suitable for high pressure or phase change refrigeration may pose risks when used within a blood vessel. Accordingly, there is a need for an improved catheter construction for cryogenic angioplasty.
Another factor which adds complexity to the task of cryocatheter design is that the primary mechanism of treatment involves thermal conduction between the catheter and a targeted region of tissue. Thus, not only is the absolute cooling capacity of the catheter important, but the nature and extent of contact between the cooled region of the catheter and the adjacent tissue is important. Effective contact may require moving, positioning, anchoring and other mechanisms for positioning, stabilizing and changing the conformation of the cooled portion of the catheter. Slight changes in orientation may greatly alter the cooling range or characteristics of the catheter, so that even when the changes are predictable or measurable, it may become necessary to provide positioning mechanisms of high stability or accuracy to assure adequate treatment at the designated sites. Furthermore, it is preferable that a vessel be occluded to prevent warming by blood flow during treatment. Beyond that, one must assure that the cooling activity is effective at the surface of the catheter, and further that defects do not cause toxic release of coolant or dangerous release of pressure into the body.
Secondary environmental factors, such as the circulation of blood near or at the treatment site may also exert a large influence on the rate at which therapeutic cooling accrues in the targeted tissue.
There is therefore a need for improved cryocatheter constructions to occlude blood flow and form a dependable thermal contact with a vessel wall.